Camshaft Torque Actuated (CTA) Variable Cam Timing (VCT) devices rely upon the camshaft torque caused by cylinder valve lift events to adjust the camshaft timing of an engine. When toque actuated cam phasers are used in conjunction with valve deactivation systems such valve lift may not occur, the resulting decrease or in some instances absence of camshaft torque may prevent reliable actuation of the camshaft phaser. As a result, desired camshaft timing adjustment may not be achieved during valve deactivation.
U.S. Pat. No. 7,255,077 discloses a cam phaser that adjusts the cam timing of a valve. However, if the cam phaser were to be used in conjunction with a valve deactivation device, the phaser may be rendered inoperable due to the reduction of cam torque. Consequently, both valve timing and valve deactivation could not be synchronously performed in such an engine, thereby reducing engine efficiency.
Recognizing the problems mentioned above and in an attempt to resolve at least some of the problems the inventors developed a variable cam timing system in an engine. The variable cam timing system includes a camshaft receiving rotational input from a crankshaft. The camshaft includes a valve cam rotationally actuating a valve coupled to a cylinder and a null cam actuating a null follower including a null spring exerting a return force on the null cam during interaction between the null cam and the null follower, where the null follower is independent from the cylinder. In this way, a follower that is not associated with valve actuation may be used to generate camshaft torque. As a result, a cam phaser coupled to the camshaft may be operated over a wider range of engine operating conditions thereby increasing engine efficiency.
Further in one example, the null follower may be selectively engaged and disengaged. For instance, the null follower may be activated responsive to deactivation of the valve. Consequently, the system's efficiency may be improved by providing additional camshaft torque only when desired to reduce losses caused by the interaction between the null cam and the null follower.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.